In the image forming apparatus that uses an electrophotographying method such as a copying machine, a printer, a facsimile and a multi-function peripheral equipped with many of these functions, a latent image corresponding to a document is formed onto a photoconductive drum; a toner is applied to this latent image to visualize this toner image; this toner image is transferred onto the recording material; and the toner image is fixed onto the recording material.
In addition, in case of forming a color image, after the latent images of Y, M, C and K corresponding to a document color are formed onto four photoconductive drums and the toner images of four colors is primarily transferred onto an intermediate belt, the toner image transferred onto the recording material is fixed and the toner image transferred onto the recording material is ejected.
As described above, as a fixing apparatus for fixing a toner image, there is provided a fixing apparatus for heating/pressing the recording material, onto which a toner image has been transferred while nipping and transferring the recording material by a nip portion formed between a fixing roller, which contains a heating device, such as a halogen lamp, and a pressing roller, which presses the fixing roller. Since structure is simple, such a fixing apparatus is widely used.
By the way, in order to attain improvement in the speed, it is necessary to supply sufficient quantity of heat for the recording material with which the toner was transferred, and in such fixing apparatus, it is necessary to make a nip width wide for the purpose. In order to make the nip width wide, it is feasible to increase the load of the pressing roller for pressing the fixing roller, the thickness of the elastic layer formed from the silicone rubber in the pressing roller and the diameter of two rollers.
However, when the load of the pressing roller and the thickness of the elastic layer are increased, there is a possibility that the nip width of an axial direction may become uneven and the crease of the recording material and fixing unevenness may occur. In addition, when the diameter of the roller is increased, the fixing apparatus is not only enlarged, but there is a problem that warming up time becomes long.
In order to solve this problem, there has been disclosed in the Official Gazette, a fixing apparatus of a so-called FBNF (Free Belt Nip Fuser) system having a fixing roller, which has an elastic layer formed from silicone rubber and contains a heating device, such as, a halogen lamp in the center, a fixing belts of an endless style, which is driven and rotated by the fixing roller and a pressing pad, which presses the fixing roller through the fixing belt from the inside of the fixing belt, which heats and presses the recording material, onto which the toner image has been transferred, in the nip portion which is formed by the fixing roller and the fixing belt. (For example, Unexamined Japanese Patent Application Publication No. 2004-206105).
According to this fixing apparatus, the fixing belts, which was pressed by the pressing pad, is elastically deformed and a broad nip portion is formed between the fixing roller and the fixing belts. Therefore, it can respond to the improvement in the speed, and fixing apparatus is not enlarged. Furthermore, since the heat capacity of fixing belts is small, warming up time is shortened and it becomes energy saving.
Here, solid rubber is used as a pressing pad from a viewpoint of the needed load for fixing and durability, but sponge is not used.
In addition, it is necessary to make the pressure of the pressing pad into a uniform pressure distribution as a whole in a nip portion. When pressure distribution is not uniform, the fixed luminous intensity and density of an image will become uneven, an image gap will occur, or a crease is formed on the recording paper as a recording material.
On the other hand, the recording material, which has passed through the nip portion, may stick to the fixing roller by the adhesive power of melted toner. In order to prevent this, it is desirable to use a high pressure only in the end of the nip portion, and to carry out elastic deformation of the fixing roller with big curvature.
Thus, it is necessary that the pressure distribution of the nip portion is set high only in the end of the nip portion, and the pressure distribution to the others is set uniform. For this reason, it is necessary to structure the pressing pad into the form, which has such a pressure distribution.
With respect to processing methods of the solid rubber, which form a pressing pad, there exist a polishing process and molding by a metal mold.
In the case of the polishing process, when the solid rubber is limited to the rubber of high hardness and there is a size step by processing, big pressure differences will arise in the portion, and a fixing image will be influenced. In addition, when fixing an image onto the recording material with narrow width using the pressing pad formed by the rubber of high hardness, the step produced in the pressing pad by the edge remains without disappearing immediately. Then, when the image is fixed onto a wide recording material, there is a possibility that the step may appear in an image.
In molding by a metal mold, there is no restrictions of the hardness of rubber, and its degree of freedom of shape selections increases. However, it is easy to produce the variation in a size or hardness between one end of the longitudinal direction and the other end with temperature distribution, a cure rate and transfer pressure at the time of molding.
In the conventional fixing apparatus disclosed in Unexamined Japanese Patent Application Publication No. 2004-206105, since a pressing pad and an elastic pressing member are formed with one rubber, whether the process is a polishing process or molding by a metal mold, the problem described above arises. As a result, there is a possibility that a desired pressure distribution could not be easily realized in the nip portion, the luminous intensity and the density of the image, which has been fixed, may become uneven, an image gap may occur or a crease may be formed on the recording paper as a recording material. In addition, the reliability of apparatus was spoiled while becoming a high cost, since the yield in processing of the pressing pad was bad, and an inspection and adjustment took time.
An object of the present invention is to provide a belt fixing apparatus and an image forming apparatus equipped with the belt fixing apparatus, which are arranged to obtain a desired pressure distribution in a nip portion by forming a pressing pad and a pressing member as an elastic pressing member from a plurality of members to solve the problems described above.